Vertical risers have been known and used in the past for transporting hydrocarbon fluids, including oil and natural gas, from subsea manifolds and well heads to production vessels on the surface. A vertical riser used for this purpose gives rise to a fundamental problem of station-keeping on the part of a production applied to a vertical production riser by tension devices to keep the riser from failing due to buckling or bending, especially in deep water. The tension force on a vertical riser can increase to relatively high values, such as up to 600 kips in times of heavy weather. Also, the relative high tension requires the use of heave compensators and further require frequent inspection and replacement of segments of the riser due to fatigue.
Another undesirable feature related to the use of a vertical production riser is the wear problem on the joint connecting the lower end of the riser to a subsea manifold. In heavy seas or other emergency situations, it is oftentimes necessary for a riser to be separated from its connection near the seabed. This causes a recoil of the riser which, along with bending of the separated pipe, can cause damage to the pipe and to the production vessel at the upper end of the riser.
Other limitations involved with the use of a vertical riser include the preference for a semisubmersible production vessel to cause a reduction in the wave-induced motions of the production vessel at the expense of deck load capacity; the need for increasing the restraint on the production vessel to keep it within a limited watch circle as riser length is increased; and the need for an elaborate control system for a storage tanker adjacent to the production vessel to avoid the lateral loading of production vessel and riser due to the presence of the storage tanker.
Because of the above drawbacks, a need has arisen for improvements in the construction and configuration of risers for transporting hydrocarbon fluids from subsea manifolds to production vessels on the surface. The present invention is directed to an improved riser for satisfying this need.
A pertinent patent related to the method of laying submarine pipelines is U.S. Pat. No. 3,266,256, issued Aug. 16, 1966.